The present invention relates to a controllable filter for being interposed between an antenna on the one side and a receiver and a transmitter on the other side, such as a frontend filter for a communication device, for example a radio telephone apparatus.
Recently, telephone systems that combine cellular telephone capabilities and cordless telephone capabilities have been proposed. Such an arrangement is schematically shown in FIG. 5. As can be seen in FIG. 5, the cordless telephone system (CTS) consists of a so-called fixed part FP and a mobile hand-set MS. Communication between the fixed part PP and the mobile hand-set is conducted along an uplink band, which is associated with the transmitter TX of the mobile part and the receiver RX of the fixed part. Equally, a down-link band is provided, which is distinct from the up-link band, with which the transmitter TX of the fixed part FP and the receiver RX of the mobile part is associated.
Furthermore, the mobile part MS can also communicate with the base transceiver station BTS that belongs to a cellular network. Such a dual-mode hand-set will typically be controlled to communicate with the fixed part of the cordless system when in range of said fixed part, and otherwise to communicate with the cellular system. In this way, a user of the hand-set can take advantage of the lower costs associated with the cordless phone that is attached to the PSTN (public switched telephone network) when in range of the fixed part of the cordless phone, and otherwise to nonetheless always have an operable telephone, at least as long as the user is in range of a base transceiver station BTS.
Regarding the communication bands for the base transceiver station BTS and the mobile hand-set MS, these correspond exactly to those between the mobile hand-set MS and the fixed part FP of the cordless telephone system, as can again be seen from FIG. 5. It may be noted that the term xe2x80x9cup-linkxe2x80x9d in the context of this application describes communication from the mobile device, and the term xe2x80x9cdown-linkxe2x80x9d describes communication to the mobile device.
In the above described type of system, a close monitoring and sophisticated selection of operating frequencies is required, such that the communication between a given mobile station MS and a cordless fixed part does not interfere with the communication in the cellular system. Conventionally, this type of frequency management is handled by the mobile station, which receives corresponding information from the base transceiver station, as indicated by a dashed line arrow along the down-link from the base transceiver station to the mobile station in FIG. 5. The mobile station then correspondingly informs the fixed part, as indicated by a dashed line arrow along the uplink between the mobile station and the fixed part.
This conventional approach, however, has a problem in that interference can nonetheless occur, namely if a rearrangement of the cellular frequencies takes place while there is no mobile station logged on to the cordless telephone system, and consequently the fixed part of said cordless telephone system cannot be informed of the rearrangement. In this case, the fixed part will continue to transmit on the same beacon channel (which is the CTS equivalent of the broadcast control channel BCCH in GSM) and thereby possibly interfere with the traffic on the cellular network.
It is therefore desirable to enable the fixed part FP of the cordless telephone system to also receive information from the base transceiver station. One possibility of achieving such a communication is indicated in FIG. 6, in which the fixed part receives an additional receiving capability in the down-link band to thereby be able to also receive downlink transmissions sent out by the base transceiver stations.
It is the object of the present invention to provide a device for enabling the above described type of communication, which at the same time is low in cost and efficient.
This object is solved by a controllable filter for being interposed between an antenna on the one side and a receiver and a transmitter on the other side, having a first terminal leading to the antenna, a second terminal leading to the receiver and a third terminal leading to the transmitter, wherein the controllable filter is capable of being controlled to selectively provide at least a first and a second filter characteristic with respect to frequency between said first terminal and said second terminal.
The basic structure of the invention is shown in FIG. 8, in which 1 denotes the controllable filter, 2 an antenna, 3 a receiver, 4 a transmitter, 12 the first terminal or antenna terminal, 13 the second terminal or receiver terminal, 14 the third terminal or transmitter terminal, 5 a control circuit and 51 a control signal.
As can be seen, the basic concept of the invention consists in splitting the signal from or to the antenna along three paths of filtering characteristics, where two of said three paths are recombined. More specifically, as can be seen, the control signal 51 determines if the uplink path between terminals 12 and 13 or the downlink path is selected, where each path has its own frequency characteristic. In this way, e.g. a fixed part of a cordless telephone using such a filter as a frontend filter has a reception and transmission capacity in the downlink band, and also retains its reception capability in the uplink band, but nonetheless only requires one receiver and one transmitter. In other words, in a communication device using the above mentioned filter, a simplex operation (listen mode) can be provided, such that no additional receiving or transmitting means are necessary.
It should be noted that the above described filter device is not restricted to being applied in the fixed part of a cordless telephone system. Much rather, the application to any communication system that may advantageously employ the above mentioned simplex listening operation is possible. For example, another advantageous application of the controllable filter of the present invention is in mobile hand-sets that function in accordance with any given mobile standard, where the use of the filter of the invention as a front-end filter allows the providing of a walkie-talkie function, i.e. two hand-sets may directly communicate with one another without the intervention of the cellular system. This is again achieved by not adding a second receiver and second transmitter to the mobile station, but by letting the mobile station aquire receiving and transmission capability in one common band, for instance by a second receiving path in the uplink band.